Why CO2 Sinks In A Sauna.

Why CO2 Sinks In A Sauna.

But it will rise again; for you breathe over and over, if you let it.

There is something any person who builds a sauna should know about the air we breathe, and CO2 is a part of that. Fresh air is Nitrogen 78.08 %, Oxygen 20.95%, Argon 0.93%, and other gases make up around 0.04%.

CO2 is part of the other gases — it is gaining slowly and changing the mix ratio slightly since the beginning of 1000 to present, we have gone from 280 ppm (parts per million) background to 465 ppm. At around 1000 ppm, the human starts to see cognitive effects. If ventilation is done incorrectly in a sauna, the PPM can exceed toxic levels.

That is why I feel it is so important to educate people on the right and wrong way to ventilate a sauna. It can be deadly or brain-damaging if you don't do it right. See Sauna Thermodynamics for recommended venting.

The thing about CO2 is that it is slightly heavier than air, but the density is so similar that it mixes very well. We have one very good advantage going for us: the density difference due to temperature.

Do you understand the concept of a hot air balloon? You know the big burner forces hot CO2 into a balloon, and the balloon rises. The keyword is hot. Because the CO2 is much hotter than the surrounding air, the balloon floats up and away. This is a result of hot gases having lower densities than cold air; Hotter gases = Rising, Colder = Falling, simple as that.

When the air in the balloon cools and is not replaced with hot CO2, it falls by gravity.

Now think about this. And look at the cold ghostly white wisps of CO2 coming off dry ice placed in a container of water, and how the wisps fall to the ground. The keyword is cold.

If CO2 is hotter than the air surrounding it, the CO2 will rise. If CO2 is colder than the air around it, the CO2 will fall.

In a sauna, the CO2 is at the same temperature as the human body. It is being expelled at 98 F into air at 150–190 F, much hotter than 98 F.

When you apply what you know or have just learned, it is easy to see that CO2 absolutely falls to the floor in this environment.

Fact: In the Sauna environment, CO2 is 98 F, and the fresh air is 150- 190 F; it is denser and falls to the floor.

Not getting rid of it properly on the floor is the biggest error people make.

Since the CO2 falls to the floor, it is easy to get rid of, and we can get a bonus from eliminating it this way.

Again, this is where people screw it up.

Two critical considerations are introducing fresh air correctly and removing stale air before it can mix with the fresh.

If CO2 mixes with the fresh air and is reheated, it will rise, and you will be recycling it.

If the Sauna is wood-fired, the stove itself handles this wonderfully. In the process of combustion, the stove creates a strong draft. This stove will create a vacuum in the sauna of between 40 and 60 cubic feet per minute (CFM). EPA 28A: No fan is needed, but often people do install a booster fan on the inlet air to increase the fresh air pressure under the stove and around the edges of the outside shroud. This creates an air dam that keeps stale air from circulating under the stove and being recirculated. It is very important.

The traditional crack under the door is something that apparently a finish person did many centuries back and passed it down to become an urban legend in sauna ventilation design. It can be used correctly if certain conditions are met. If not, then it works against proper ventilation. let me explain.

If the stove is the only source of elimination, it creates a vacuum when wood is burned inside, and smoke is eliminated outside the building via the smoke stack.

If it is in a sealed room, the fire will go out, and no draft will form because the fire needs air. Where do we give the stove this needed air?

You have two choices or a combination of choices. The best way is to make sure it works for us, not against us.

The proper place to bring air into the Sauna is from outside to a point directly under the stove, as most stove manufacturers state in the instructions.

This creates slight air pressure as the airflow packs in under the hot rocks and keeps pushing air out below the stove on all sides as it rushes to meet the demand of the stove. The hot rocks heat the fresh air, and it flows to the breathing area.

The stove is sucking in all the stale air and the required air needed for combustion.

So what does the big gaping crack under the door do? That depends. If the air flow is out of the sauna because it is on the lee side of the building, it helps, but only in that situation. If there is a strong wind pushing air in, it is mixing with the stale air, short-circuiting where the air is supposed to enter, gets sucked under the stove, and goes into the breathing chamber.

If the air is moving out of the sauna under the door, then more air has to be coming in. If it is coming in under the stove, then there is more circulation within. This increased circulation is creating a convection oven effect, or forcing the temperature to be higher at lower elevations. Just as a convection oven does. This is good. I am not sure this can be done with natural aspiration; it most probably requires a fan on the fresh air side to achieve this.

I do not have a gap under my door because it faces the prevailing breeze and would short-circuit my ventilation. I am not against it, just as long as it is done correctly and tested. I do have a vent to the outside that is like the gap under the door, but it can be regulated to work with the breeze or shut off depending on the conditions. In most cases, it is completely unnecessary as the stove does the job.

Currently, saunas are a big fad in America, and people are building them who really don't understand ventilation and CO2, how or why. I write this to educate people that ignorance of these things can be undesirable and dangerous. My heart is with the do-it-yourselfer like myself because it is the most Finnish spirit thing there is about sauna.

Please consider how your ventilation will work precisely and test it, making sure you design it so that it actually works the way you think it works.